The term goal of this project is to produce a comprehensive and detailed understanding of the cellular and molecular mechanisms that guide formation of peripheral nerves in vertebrates. During development, numerous connections are established between the central nervous system (CNS) and the body via peripheral nerves. These nerves are responsible for conveying information in two directions. Some nerves carry signals out of the CNS to the body to control movement and autonomic functions whereas other nerves transmit information about external stimuli from the body to the CNS. Failure to properly form or maintain peripheral nerves results in a large array of peripheral neuropathies that are manifested as, for example, muscle weakness and loss of muscle control, gastrointestinal dysfunction, inability to sense pain or change in temperature and neuropathic pain. Our approach to investigating peripheral nerve development is to create transgenic zebrafish that express fluorescent proteins in different cellular components of peripheral nerves and then combine in vivo, time-lapse imaging with tests of gene function. In preliminary studies we determined that perineurial cells, which form a protective sheath around myelinated motor nerves, originate as glial cells in the ventral spinal cord and that they are essential to motor nerve development. In Specific Aim 1 of this project we will investigate the molecular mechanisms that specify ventral spinal cord precursors to develop as perineurial cells. In Specific Aim 2 we will investigate the roles of perineurial glia and Schwann cells in motor nerve formation. In Specific Aim 3 we will characterize mutations that disrupt Schwann cell development and myelination as a way to identify genes that are necessary for motor nerve formation. Completion of these aims will greatly extend our understanding of both cellular behavior and gene function in peripheral nerve development, facilitating effective design of therapy intended to treat peripheral neuropathies.